The present invention relates to a fixing apparatus for heat-fixing an unfixed toner image in an imaging apparatus utilizing eletrophotography in copying machines, printers, facsimiles and the like, and more particularly to a belt-nip type fixing apparatus.
There are known fixing apparatus for fixing an unfixed toner image carried on a recording sheet by heating-melting such a toner image. A typical fixing apparatus is designed to fix a toner image by forcing a pressure belt capable of endless movement to be contact a heat-fixing roll rotatably supported, and feeding a recording sheet between them. For example, Japanese Unexamined Patent Publications Nos. 69337/1977, 151677/1985, 151681/1985, 14675/1987, Japanese Unexamined Utility Model Publication Nos. 104852/1985, 30961/1990 (No. Hei 2-30961) and Japanese Unexamined Patent Publications Nos. 50885/1992, 150679/1993 disclose fixing apparatus of the sort mentioned above.
FIG. 7 shows a fixing apparatus proposed by the present patent applicants and disclosed in Japanese Unexamined Patent Publication No. 150679/1993. A heat-fixing roll 101 used in this fixing apparatus has a cylindrical core 112 of metal such as aluminum offering a high thermal conductivity and a resilient layer 120 formed on the surface of the core. The resilient layer 120 is formed with a primary layer 113 of HTV silicone rubber for directly covering the surface of the core, and a topcoat 114 of RTV silicone rubber for covering the primary layer.
A halogen lamp 107 as a heat source is placed inside the core. Further, a temperature sensor 111 in contact with the surface of the heat-fixing roll 101 is installed and used for measuring the temperature of the surface of the resilient layer. A temperature controller (not shown) operates to control ON/OFF of the halogen lamp 107 according to a measurement signal from the temperature sensor 111, so that the temperature of the surface of the heat-fixing roll 101 is adjusted to a predetermined level. Moreover, a parting agent is supplied by an oil supply unit 110 to the surface of the heat-fixing roll 101, whereby part of an unfixed toner image 116 is prevented from being offset-printed on the heat-fixing roll 110 when the unfixed toner image 116 is fixed on a recording sheet 115.
The pressure belt 102 is stretched along support rolls 104, 105 and a pressure roll 103, and the pressure roll 103 is forced to be contact with the heat-fixing roll 101, so that part of the pressure belt 102 is in contact with and wound around the heat-fixing roll 101. Further, an auxiliary pressure roll 106 is pressed against the heat-fixing roll 101 via the pressure belt on the upstream side of a portion where the heat-fixing roll 101 and the pressure belt 102 contact each other. When the heat-fixing roll 101 is driven to rotate, the pressure belt circumferentially rotates in direction of arrow in FIG. 7, and the aforementioned contact portion forms a nip through which the recording sheet 115 carrying the toner image 116 is passed. When the recording sheet 115 carrying the unfixed toner image is fed into the nip, it is held between the heat-fixing roll 101 and the pressure belt 102 before being conveyed further. Toner is then melted by the heat transmitted from the heat-fixing roll 101 and forced by the press contact force of the pressure belt 102 or the pressure roll 103 to stick to the recording sheet 115.
The adoption of the belt-nip type fixing apparatus like this is advantageous in that the recording sheet conveyance speed is increased while fixing time is sufficiently secured in comparison with an apparatus in which a heat-fixing roll and a pressure roll are forced to be contact with each other without using a pressure belt, since the recording sheet is continuously fed to cover the length of the belt nip (the length in the range in which the pressure belt is kept in contact with the heat-fixing roll) and is also continuously heated. As the heating time in the belt-nip type is longer than that in any other type without using the pressure belt provided the conveyance speed is equal, moreover, a great deal of heat can be transmitted by means of toner. Accordingly, the belt-nip type fixing apparatus is specifically fit for use in a color copying machine in which multilayer toner is to be colored as desired.
In the fixing apparatus, further, the resilient layer 120 is formed on the surface of the heat-fixing roll 101. The resilient layer 120 is deformed on receiving the press contact force of the pressure roll so as to be slightly distorted in the circumferential direction. In other words, a portion of the resilient layer 120 that the pressure roll is forced to be contact is distorted as the heat-fixing roll 101 turns and that portion is set free from distortion after the heat-fixing roll 101 turns further. When the heat-fixing roll 101 is driven to rotate at a peripheral velocity of V0 in a portion without deformation, the peripheral velocity V1 in the portion of the pressure roll producing distortion .epsilon.1 in the circumferential direction is expressed by the following equation: EQU V1=V0(1+.epsilon.1)
The phenomenon in which the peripheral velocity of the heat-fixing roll in the portion that the pressure roll contacts occurs likewise even when the leading end of the recording sheet passes through the belt nip, and a slight deviation develops between the recording sheet 115 fed substantially at a velocity of V0 and the surface of the resilient layer 120. Therefore, the adhesion of the toner image 116 to the heat-fixing roll 101 is canceled and the recording sheet 115 strips off the heat-fixing roll 101. As the adhesion of the molten toner to the surface of the heat-fixing roll 101 is affected by the physical property of the chemical materials of both on the interface, the behavior of the recording sheet 115 which tends to strip off differs according to the kind of toner and the material quality of the resilient layer 120. In this fixing apparatus, however, the recording sheet can be stripped off the heat-fixing roll 101 without using a peel pawl normally employed in a fixing apparatus comprising a fixing apparatus and a pressure roll (hereinafter called self-stripping). In this fixing apparatus, moreover, thin paper difficult to strip off because of so-called weak nerve and paper with a large quantity of toner sticking thereto can also be self-stripped.
In order to ensure the self-stripping like this, the value of distortion .epsilon.1 in the circumferential direction is ought to be increased to a certain degree. When, however, great press contact force is applied to the pressure roll 103 to secure such distortion, the frictional force between the recording sheet 115 and the heat-fixing roll 101 in that portion increases, and the conveyance speed Vp of the whole recording sheet may become almost equal to the speed V1 of the portion where the distortion has developed. Then the peripheral speed of the heat-fixing roll 101 on the upstream side of the press-contact position of the pressure roll 103 substantially moves to V0 (the peripheral speed of the portion where no deformation is produced). The problem in this case is that the difference between the speed Vp of the recording sheet and the peripheral speed V0 of the heat-fixing roll causes a deviation on the contact face, which results in image aberration.
In consideration of the above problems, the apparatus disclosed in Japanese Unexamined Patent No. 150679/1993 is provided with the auxiliary pressure roll 106 disposed on the upstream side in the travel direction of the recording sheet with respect to the pressure roll 103 so as to press the auxiliary pressure roll 106 against the heat-fixing roll 101. Even when the leading end of the recording sheet 115 reaches the press contact position of the pressure roll 103 to exert the force of conveying the recording sheet at a speed almost equal to V1, the following portion of the recording sheet is pressed against a portion of the heat-fixing roll 101 moving substantially at the peripheral speed V0 to cancel the difference in speed between the heat-fixing roll 101 and the recording sheet 115. Image aberration is thus avoided.
When, however, the auxiliary pressure roll 106 is strongly pressed, compressional deformation is caused to the resilient layer of the heat-fixing roll 101 even in the portion against which the auxiliary pressure roll 106 is pressed as shown in FIG. 8, and distortion circumferentially develops in the peripheral surface in that portion.
When such a distortion develops, the peripheral velocity of the heat-fixing roll 101 becomes V3, which is greater than the peripheral velocity V0 of the portion without deformation, as in the press-contact position of the pressure roll 103 so as to exert the force of conveying the recording sheet at a speed greater than V0. Therefore, the frictional force base on the press contact force of the pressure roll and what is based on the press contact force of the auxiliary pressure roll make the recording sheet conveyance speed Vp almost equal to V1 or V.sub.3. Consequently, there is produced the difference between the recording sheet conveyance speed Vp and the peripheral velocity V of the heat-fixing roll in the portion between the press-contact portion and the auxiliary pressure roll (an area A shown in FIG. 8), which results in image aberration.
In the case of a fixing apparatus in which pressure rolls for use in stretching a pressure belt along them is forced to be contact with a heat-fixing roll, on the other hand, there is a problem arising from ruining the luster of the image already fixed on one side of a recording sheet at the time another image is fixed on the other side when it is attempted to fix toner images successively on both sides thereof. The reason for this is considered attributable to the fact that the pressure roll is kept in press-contact with the heat-fixing roll having a built-in heating means via the pressure belt and is always heated at high temperatures. More specifically, the toner image already fixed on the one side of the recording sheet is heated again by the heat form the pressure roll when the image on the other side thereof is fixed, and the luster of the image on the one side is considerably changed as the toner image melts. When the toner image on the one side of the recording sheet melts, the trace of the joint of the pressure belt may be left thereon and in a case where the recording sheet is fusion-bonded to the pressure belt, the problem is that the recording sheet is not readily stripped off.